Reference voltage source

ABSTRACT

Method for uniformly charging a photoconductive insulating surface. A nonuniform charge density is formed on the photoconductive insulating surface and is thereafter contacted with an insulating material causing the charged surface to attain a uniform charge density in contacted areas.

United States Patent Leiga 1151 3,648,133 1 Mar. 7,1972

[ UNIFORM ELECTROSTATIC CHARGING OF A PHOTOCONDUCTIVE INSULATING SURFACE v V [72] Inventor: Algird G. Lciga, Pittsford, N.Y.

[73] Assignee: Xerox Corporation, Rochester, N.Y.

[22] Filed: Mar. 31, 1970 [21] Appl. No.: 24,288 J 3,546,545 12/ I970 Sato et al. ..3l7/262 A 3,549,962 12/1970 Roth et al. ..3l7/262 A Primqry'Exqrniner Gerald Goldberg Assistant Examiner-Harry E. Moose, Jr.

Anomey--James J. Ralabate, Irving Keschner and John E. Beck Method for uniformly charging a photoconductive insulating surface. A nonuniform charge density is formed on the iphotoconductive insulating surface and is'thereafter con- 1 tacted with an insulating material causing the charged surface to attain a uniform charge density in contacted areas.

- 15 Claims, 5 Drawing Figures Patented March 7, 1972 FIG. IA

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INVENTOR ALGIRD G. LEIGA ATTORNEY REFERENCE VOLTAGE SOURCE BACKGROUND OF THE INVENTION In the process of xerography, for example, as disclosed in Carlson U.S. Pat. No. 2,297,651, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with electrostatically charged, finely divided material, such as an electroscopic powder, that is brought into surface contact with the photoconductive layer, the powder depositing in a pattern corresponding to the latent electrostatic image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means. I

In the practice of the xerographic process as described in the preceding paragraph, there has been a need for charging the photoconductive insulating layer to a uniform potential on the order of several hundred volts. It has been usual to charge the plate with corona of positive polarity by means of a corona-generating device, which when supplied with potential above the corona threshold produces an emission of corona ions. Typical embodiments of corona-generating devices are disclosed in the Walkup U.S. Pat. No. 2,777,957 and in Vyverberg U.S. Pat. No. 2,836,725.

As indicated above, it is customary when charging a xerographic plate, especially a selenium xerographic plate, to apply corona of positive polarity. This is because it has been found that while selenium can conduct both electrons and holes the mobility for holes in approximately times that for the electrons. Accordingly, negative corona charging and dissipation thereof in response to an optical pattern is normally insufficient to properly discharge the photoconductive layer. Rather, the electrons are trapped within the bulk of the photoconductive layer establishing a space charge which prevents effective reproduction of the original image.

Xerographic plates have been developed, on the other hand, which are capable of accepting corona of negative polarity and dissipating such charge in response to an optical pattern. Such plates have a sufficiently high mobility for electrons to substantially dissipate the charge in illuminated area. Exemplary plates are those disclosed by Greig U.S. Pat. No. 3,052,539 and by Middleton et al. U.S. Pat. No. 3,121,006.

It has been found, however, that use of the conventional corona charging devices for negative charging results in a nonunifonn charge distribution which is not totally acceptable for xerographic purposes. Negative corona has a tendency to concentrate itself at discrete points along the corona wire with result that negative charge deposits on the photoconductive insulating layer in a nonuniform pattern corresponding to the nonuniforrnity of the corona discharge itself, while positive corona discharge from a wire generally occurs in a volume in the form of a continuous uniform sheath surrounding the wire. Designs correcting for this deficiency provide complicated and/or expensive apparatus which is normally undesirable because of its complex nature.

SUMMARY OF THE INVENTION The present invention provides a method for uniformly negatively charging a photoconductive insulating surface. In particular, a photoconductive insulating surface is negatively charged, such as by conventional negative corona charging, the charged surface thereafter being brought into contact with an insulating material capable of causing the charged surface to attain a uniform negative charge density.

It is an object of the present invention to provide an improved method for negatively charging a photoconductive insulating surface uniformly.

It is a further object of the present invention to provide an improved method for negatively charging a photoconductor insulating surface uniformly wherein an insulating liquid is applied to a nonuniformly, negatively charged photoconductive insulating surface, the surface thereafter attaining a uniform charge density.

It is still a further object of the present invention to provide a simple and economical technique for negatively charging the surface of a photoconductive insulator uniformly.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:

FIGS. lA-lE are a diagrammatic illustration of the basic flow steps in the process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In uniformly redistributing the charge on a photoconductive insulator, it has been found that one must use an insulating liquid of sufficiently high vapor pressure which has, at least, slight solvent properties for a thin surface film existing over the photoconductive insulator layer. For a photoconductive insulating layer fabricated as a binder structure, the binder matrix polymer may serve as the thin surface film. By sufficiently high vapor pressure" as used in this specification and claims appended thereto, it is meant that the insulating liquid will evaporate under ambient conditions without further inducement or it can be caused to evaporate by the gentle application of heat, said heat being insufficient to adversely affect the layer which is being uniformly charged. In any event, the material should not have such a high vapor pressure that it will not uniformly distribute the charge on the surface prior to complete evaporation. In the production of a selenium xerographic plate, for example, after deposition of the amorphous selenium layer, a break-in process follows wherein the selenium layer is surface treated, as for example, with cascading toner polymer. Exemplary toner polymers include various blends of styrene with normal or isobutyl methacrylatc. This break-in process which causes an extremely thin layer of polymer to be coated over the selenium layer has been found necessary in the production of a xerographic plate having a normally desirable physical characteristic. In accordance with the present invention, it has been found necessary to use an insulating liquid which is a solvent for this thin film of toner polymer deposited over the photoconductive insulator layer if one wishes to achieve a uniform negative charge redistribution.

By having at least slight solvent action, at least that portion of the toner polymer film having the electrostatic charge thereon is brought into solution and uniformly scattered throughout by electrostatic repulsion of charges to like polarity. When the liquid is evaporated the electrostatic charge associated with the toner polymer will be uniformly deposited on the photoconductive layer as the polymer comes out of solution and uniformly deposits on the layer. If the liquid does not have the required solvent properties, this electrostatic repulsion between the charges will not take place with the result that uniform charge redistribution is not attained.

Applicable insulating liquids are chosen based upon the photoconductive insulator and the polymer film thereon upon which a uniform charge density is to be produced. A typical insulating liquid is chloroform, although aromatic hydrocarbons, ketones and esters may be utilized as long as they are not too conductive.

Referring now to the drawings, FIG. 1A shows the electrostatic charging of imaging member 10 comprising a conductive support member 11 having a layer of photoconductive insulating material 12 thereover. Conductive support member 11 may comprise such materials as aluminum, brass or other metals, metallized paper or paper with a relatively high moisture content, glass with a transparent or other conductive coating, or like known layer. Imaging member is negatively electrostatically charged by moving relative thereto a corona charging device 13 which is connected to a negative high-voltage power supply 14. As previously indicated, corona charging devices are well known in the art and suitable ones for use in the practice of the present invention are described, for example, in U.S. Pat. Nos. 2,777,957 and 2,836,725. It should be noted that the invention also may be utilized when the imaging member is positively charged in order to ensure uniform positive charging. FIG. 1B shows the nonuniform charge distribution caused by the negative corona charging of FIG. 1A.

FIG. 1C shows the disposition of an insulating liquid 15 onto the photoconductive insulating surface. For example, insulating liquid 15 may be dispensed manually from a test tube 16. As set forth hereinabove, insulating liquid 15 has slight solvent properties for the toner polymer film. FIG. 1D shows the uniform redistribution of the negative charges prior to liquid evaporation. FIG. 1E shows the insulator layer having the uniform charge distribution after the evaporation of the insulating liquid. To speed evaporation, gentle heat can be applied to the underside of member 10, or hot air flowed over the top side, such heat being insufficient to adversely affect layer 12. Imaging may proceed according to well-known xerographic techniques including illumination to a pattern of light and shadow whereby charges in illuminated areas will be dissipated in accordance with the radiation intensity received. The latent electrostatic image which remains after image formation can be developed into a visible image by standard development techniques.

DESCRIPTION OF SPECIFIC EMBODIMENTS The following examples are given to enable those skilled in the art to more clearly understand and practice the invention. They should be considered not as a limitation upon the scope of the invention but merely as being illustrative thereof.

EXAMPLE I A xerographic plate comprising a layer of cadmium sulfoselenide over an aluminum conductive backing and having a thin Lucite (DuPont) toner polymer layer thereover is corona charged to a potential of approximately -l,080 volts. As is known, this negative corona charging produces a nonuniform charge distribution. A thin film of chloroform is added to the charged surface. After evaporation of the chlorofonn, powder cloud development shows the surface potential to be uniform within 5 volts (i.e., within the potential resolution of this development scheme).

EXAMPLE II The xerographic plate of Example I is negatively corona charged to a potential of about 1 ,080 volts. Measurement of the surface charge with an electrometer indicates nonuniform charge distribution. The plate is treated as in Example I to achieve a uniform charge density. Thereafter, the charged plate is exposed to a pattern of light and shadow to produce a latent electrostatic image which is developed using a powder cloud development technique. The developed image is transferred to a copy sheet and fixed thereon. An acceptable xerographic copy is obtained.

EXAMPLE III A xerographic plate comprising a layer of selenium over an aluminum conductive backing and having a thin Lucite (Du- Pont) toner polymer layer thereover is corona charged to a potential of approximately 800 volts. As is known, this negative corona charging produces a nonuniform charge distribution. A thin film of chloroform is added to the charged surface. After evaporation of the chloroform, powder cloud development shows the surfacepotential to be uniform within 5 volts (i.e., within the potential resolution of this development scheme).

EXAMPLE IV The xerographic plate of Example III is negatively corona charged to a potential of about --800 volts. Measurement of the surface charge with an electrometer indicates nonuniform charge distribution. The plate is treated as in Example I to achieve a uniform charge density. Thereafter, the charged plate is exposed to a pattern of light and shadow to produce a latent electrostatic image which is developed using a powder cloud development technique. The developed image is transferred to a copy sheet and fixed thereon. An acceptable xerographic copy is obtained.

While the invention has been described with reference to preferred embodiments thereof, it should be understood by those skilled in the art that various changes in form and details may be made without departing from the true spirit and scope of the invention. All such modifications, etc., are considered to be within the scope of the present invention as defined by the claims appended hereto.

' What is claimed is:

l. The method of uniformly charging the surface of a photoconductive insulator to a substantially uniform potential comprising the steps of:

a. charging the surface of a photoconductive insulator to a potential, said potential being of a nonuniform density, and

b. applying a layer of insulating liquid to the surface of said photoconductive insulator subsequent to the charging thereof, thereby redistributing said surface charge to attain a uniform charge density on said photoconductive insulator surface.

2. The method as defined in claim 1 further including the step of evaporating said liquid from said photoconductive insulator surface to provide a uniform charge density after evaporation.

3. The method as defined in claim 1 wherein the photoconductive insulating surface initially has a nonuniform negative charge thereon.

4. The method as defined in claim 1 wherein the insulating liquid has a high vapor pressure.

5. The method as defined in claim 1 wherein said photoconductive insulator has been subjected to treatment by surface contact with a polymer material, said treatment causing a thin film of polymer to be coated over the exposed surface of said photoconductive insulator to enhance the xerographic properties thereof.

6. The method as defined in claim 5 wherein said insulating liquid has at least slight solvent properties for said thin film of polymer material.

7. The method of claim 6 wherein said insulating liquid has a high vapor pressure.

8. The method as defined in claim 5 wherein said insulating liquid is chloroform.

9. The method of charging the surface of a photoconductive insulator which has previously been subjected to treatment by surface contact with a polymer material, said treatment causing a thin film of polymer to be coated over the exposed surface of said photoconductive insulator to enhance the xerographic properties of said photoconductive insulator, to a substantially uniform potential comprising the steps of:

a. charging the surface of said photoconductive insulator to a potential, said potential being of a nonuniform density, and

b. applying a layer of insulating liquid to said insulator surface subsequent to the charging thereof, said insulating liquid being slightly solvent for said thin film of toner polymer, thereby redistributing said surface charge to attain a uniform charge density on said insulator surface.

10. The method as defined in claim 9 wherein the photoconductive insulator surface initially has a nonuniform negative charge thereon.

11. The method as defined in claim 9 wherein the insulating liquid is chloroform.

12. The method of negatively charging the surface of a xerographic plate comprising a photoconductive insulator layer disposed over a conductive substrate, said photoconductive insulator layer having a thin polymer film disposed over the exposed surface thereof, said method comprising causing a corona discharge device held at a negative corona discharge potential with respect to said xerographic plate to be moved relative to said plate, the charge density after said charging being nonuniform over said xerographic plate surface subjected to said corona discharge, thereafter contacting said nonuniformly charged surface with an insulating liquid having at least slight solvent properties for said thin polymer film residing on the surface of said photoconductive insulator, and evaporating said insulating liquid to provide a uniformly redistributed negative charge over those areas of said photoconductive insulator surface contacted by said insulating liquid.

13. The method as defined in claim 12 wherein the photoconductive insulator is selenium.

14. The method as defined in claim 12 wherein said insulating liquid is chloroform.

15. The method as defined in claim 9 wherein said insulating liquid is evaporated from said photoconductive insulator surface. 

1. The method of uniformly charging the surface of a photoconductive insulator to a substantially uniform potential comprising the steps of: a. charging the surface of a photoconductive insulator to a potential, said potential being of a nonuniform density, and b. applying a layer of insulating liquid to the surface of said photoconductive insulator subsequent to the charging thereof, thereby redistributing said surface charge to attain a uniform charge density on said photoconductive insulator surface.
 2. The method as defined in claim 1 further including the step of evaporating said liquid from said photoconductive insulator surface to provide a uniform charge density after evaporation.
 3. The method as defined in claim 1 wherein the photoconductive insulating surface initially has a nonuniform negative charge thereon.
 4. The method as defined in claim 1 wherein the insulating liquid has a high vapor pressure.
 5. The method as defined in claim 1 wherein said photoconductive insulator has been subjected to treatment by surface contact with a polymer material, said treatment causing a thin film of polymer to be coated over the exposed surface of said photoconductive insulator to enhance the xerographic properties thereof.
 6. The method as defined in claim 5 wherein said insulating liquid has at least slight solvent properties for said thin film of polymer material.
 7. The method of claim 6 wherein said insulating liquid has a high vapor pressure.
 8. The method as defined in claim 5 wherein said insulating liquid is chloroform.
 9. The method of charging the surface of a photoconductive insulator which has previously been subjected to treatment by surface contact with a polymer material, said treatment causing a thin film of polymer to be coated over the exposed surface of said photoconductive insulator to enhance the xerographic properties of said photoconductive insulator, to a substantially uniform potential comprising the steps of: a. charging the surface of said photoconductive insulator to a potential, said potential being of a nonuniform density, and b. applying a layer of iNsulating liquid to said insulator surface subsequent to the charging thereof, said insulating liquid being slightly solvent for said thin film of toner polymer, thereby redistributing said surface charge to attain a uniform charge density on said insulator surface.
 10. The method as defined in claim 9 wherein the photoconductive insulator surface initially has a nonuniform negative charge thereon.
 11. The method as defined in claim 9 wherein the insulating liquid is chloroform.
 12. The method of negatively charging the surface of a xerographic plate comprising a photoconductive insulator layer disposed over a conductive substrate, said photoconductive insulator layer having a thin polymer film disposed over the exposed surface thereof, said method comprising causing a corona discharge device held at a negative corona discharge potential with respect to said xerographic plate to be moved relative to said plate, the charge density after said charging being nonuniform over said xerographic plate surface subjected to said corona discharge, thereafter contacting said nonuniformly charged surface with an insulating liquid having at least slight solvent properties for said thin polymer film residing on the surface of said photoconductive insulator, and evaporating said insulating liquid to provide a uniformly redistributed negative charge over those areas of said photoconductive insulator surface contacted by said insulating liquid.
 13. The method as defined in claim 12 wherein the photoconductive insulator is selenium.
 14. The method as defined in claim 12 wherein said insulating liquid is chloroform.
 15. The method as defined in claim 9 wherein said insulating liquid is evaporated from said photoconductive insulator surface. 